The present invention is directed to processes for converting aromatic carbamates and polymeric aromatic carbamates to their corresponding isocyanates by thermolysis in the presence of a specifically defined catalyst at atmospheric or super atmospheric pressures.
Isocyanates are very useful substances as starting materials for polyurethanes. Such polyurethanes can be used in the formation of a variety of products ranging from automative parts to thermal insulation. The properties of the final polyurethane end product is to a large extent determined by the number of isocyanate groups i.e., (--NCO) present on the isocyanate starting material. For example, difunctional isocyanates do not result in crosslinking and are useful in the production of flexible polyurethane foams. Polyfunctional isocyanates result in crosslinking and consequently are useful in the production of rigid polyurethane foams. Within the class of polyfunctional isocyanates is a subclass of isocyanates, namely, polymeric aromatic polyisocyanates, which have gained market recognition and are possessed of unique properties which render them particularly adaptable for specialized end uses such as the manufacture of urethane adhesives. The term, "polymeric isocyanates" as used herein refers to a mixture of compounds containing poly alkylene or arylene poly aryl isocyanate oligomers such as poly methylene poly phenyl isocyanate (described hereinafter in more detail).
Non-polymeric aromatic isocyanates include such compounds as tolylene diisocyanate, methylene-bis-(4-phenyl isocyanate) and naphthylene diisocyanate.
A current process for preparing these nonpolymeric isocyanates, for example, tolylene diisocyanate of the formula: ##STR1## comprises nitrating toluene to form dinitrotoluene, reducing the latter with hydrogen to form the corresponding diamine and then reacting the diamine with phosgene. Thus, the aforedescribed process comprises complicated and troublesome steps, requiring the use of a large amount of highly toxic phosgene and permitting the formation of hydrogen chloride as by-product.
An alternative approach to preparing non-polymeric isocyanates involves the synthesis of carbamates from nitro compounds and subsequently pyrolyzing carbamates to form the isocyanate and an alcohol co-product.
The reaction for forming isocyanates by pyrolysis of carbamates may be shown by the following basic equation: EQU RHNCO.sub.2 R'.fwdarw.RNCO+R'--OH (1).
On thermal dissociation of the carbamate, several undesirable side reactions take place at the same time. These side reactions are: the decarboxylation reaction of the carbamate accompanying the formation of a primary amine RNH.sub.2 and an olefin or of a secondary amine RNHR as a by-product; the reaction between the produced isocyanate and the starting carbamate, permitting the formation of an allophanate as by-product; the reaction between the produced isocyanate and an amine formed as by-product permitting the formation of a urea compound as by-product; and the polymerization of the produced isocyanate, permitting the formation of an isocyanurate or a polymer as by product. The thermal dissociation reaction of equation (1) above is reversible and its equilibrium remains with the left-hand side carbamate at low temperature but is shifted to the right-hand side by heating, whereby the dissociation of the carbamate takes place. In this case, the thermal dissociation temperature varies according to the sort of carbamate and the reaction conditions. Accordingly, it is important for obtaining isocyanates advantageously from carbamates to perform the pyrolysis reaction of equation (1) selectively while inhibiting the above mentioned side and reverse reactions.
The probability of certain undesirable side reactions occurring is increased as the reaction temperature is increased and as the time during which the isocyanate product remains in contact with the components of the reaction mixture is increased. As one lowers the reaction temperature, however, the reaction rate decreases, along with the solubility of the carbamate in any solvent used in the reaction medium.
The conventional pyrolysis of carbamates can be roughly classified into reactions carried out in the vapor phase at a high temperature and reactions carried out in the liquid phase at a relatively low temperature. U.S. Pat. No. 3,734,941 discloses a typical vapor phase process wherein a carbamate is pyrolyzed at 400.degree.-600.degree. C. in the presence of a Lewis acid and the resultant vapor is separated by fractional condensation into an isocyanate and an alcohol. According to this process, for example, tolylene diisocyanate is obtained in a yield of 60% by pyrolysis of diethyl tolylene-2,4-dicarbamate of the formula: ##STR2## in the presence of ferric chloride. However, this process has the drawbacks of a low yield of the product, decomposition of the catalyst, corrosion of the reaction apparatus at high temperatures, and formation of a considerable amount of a polymer as by-product. (See also Br. Patent Spec. No. 1,247,451).
German Pat. No. 2,410,505 proposes as an improved vapor phase method, a process wherein the residence time of the reactants at 350.degree.-550.degree. C. is controlled within 15 seconds. According to this process, the yield of isocyanate is as high as 93%, although the carbamate has to be supplied in the form of powders to the reaction zone. However, a solid polymer is also formed by this process as by-product and is gradually deposited in the reactor and in the condenser during the course of sustained operation, thus making it difficult to conduct a continuous reaction. In addition, a large quantity of heat required for the endothermic pyrolytic reaction has to be supplied to the starting material within a very short period of time. This additional factor causes this process to encounter great difficulty in being adopted into practice.
Liquid phase processes were developed in an attempt to lower the reaction temperature and reduce undesirable side reactions.
For example, U.S. Pat. No. 2,409,712 discloses the pyrolysis of N-substituted carbamic esters in the liquid phase, in the presence or absence of a diluent, at temperatures of 150.degree. to 350.degree. C. under a high vacuum to distill the resulting isocyanate overhead. None of the carbamic esters disclosed include polymeric aromatic carbamates. Consequently, not only does the use of high vacuum add to the cost of the process, but the use of high vacuum if applied to the distillation of polymeric isocyanates would be ineffective due to the very high boiling points of the latter. This patent also does not disclose the use of catalysts as described herein.
In an article in the Journal of the American Chemical Society, Vol. 81, page 2138 et seq. (1959), Dyer et al show that ethyl carbanilate gives phenyl isocyanate (60-75 mole percent based on carbinilate degraded) and ethyl alcohol when heated for 6 hours at 200.degree. C. under pressure sufficiently low (60-120 mm Hg) to vaporize the alcohol but high enough to retain the isocyanate. At atmospheric pressure no phenylisocyanate is obtained, although 70 percent of the ethyl carbanilate is destroyed. At 250.degree. C. and atmospheric pressure alpha-methylbenzylcarbanilate gives major amounts of aniline, alpha-methylbenzyl aniline, styrene and carbon dioxide.
U.S. Pat. No. 3,054,819 discloses the pyrolysis of an aliphatic mono carbamate and dicarbamate esters in the optional presence of a basic catalyst such as alkali and alkaline earth metal oxides, hydroxides, carbonates and the like. The pyrolysis is conducted at subatmospheric pressures and at temperatures of 100.degree. C. to 300.degree. C. In accordance with this process the isocyanate product must be separated from the glycol ester co-product preferably by distilling isocyanate alone or in combination with the glycol ester and separating the two co-products. Either alternative is not available with polymeric aromatic isocyanates. Thus, this patent fails to disclose (1) the use of aromatic carbamates of any kind, and (2) the use of the catalysts of the present invention in conjunction with any carbamates.
U.S. Pat. No. 3,919,278 is directed to a process for preparing isocyanates wherein a mononuclear aromatic carbamate is dissolved in an inert solvent in an amount such that the total concentration of the carbamate and a product obtained by pyrolysis thereof is within a range of about 1-20 mole % and the pyrolysis of the carbamate is carried out at 230.degree.-290.degree. C. in the presence of an inert carrier used in an amount of at least 3 molar proportion to the carbamate. Polymeric aromatic carbamates are not mentioned in this patent nor is the use of the catalysts of the present invention in conjunction with any carbamates.
U.S. Pat. No. 3,919,279 is directed to a process for preparing isocyanates wherein a carbamate is dissolved in an inert solvent and brought into contact at a high temperature (i.e. 175.degree.-350.degree. C.) with a catalyst composed of a heavy metal (Mo, V, Mn. Fe, Co, Cr, Cu or Ni) or a compound thereof to effect the pyrolysis of the carbamate at temperatures of 175.degree. to 350.degree. C. The concentration of the carbamate dissolved in the inert solvent is less than 80%, by weight, e.g. between about 3 and about 80%, by weight, 3% being the lower limit of solubility of the carbamate in the solvent. This patent emphasizes the importance of maintaining the carbamate in a substantially completely dissolved state at reaction temperature during conversion to the isocyanate to minimize the formation of polymerization products such as tars or resins as well as undesirable by-products. Product alcohol is removed from the reaction mixture in the examples at atmospheric or superatmospheric pressure. The patent fails to disclose the catalysts described herein for the present invention or the thermolysis of polymeric aromatic carbamates.
U.S. Pat. No. 3,962,302 is directed to a process for producing isocyanates by thermolysis of carbamates while dissolved in an inert organic solvent and in the absence of a catalyst. Reaction temperatures range from 175.degree. to 350.degree. C. (preferably 200.degree. to 300.degree. C.) at carbamate concentrations of between 3% and 80%, by weight, of the reaction solution. This patent fails to disclose the thermolysis of polymeric aromatic carbamates and the use of catalysts of the present invention with any carbamates.
U.S. Pat. No. 4,081,472 is directed to a process for preparing aromatic isocyanates by the thermolysis of an aromatic carbamate at temperatures of 150.degree. to 350.degree. C. (preferably 200.degree. to 300.degree. C.) under substmospheric pressure in the presence of a catalyst dissolved in an inert solvent. The resultant isocyanate and alcohol must be removed in vapor form during the reaction and thereafter separately condensed (See Col. 5 lines 55 et. seq. and Col. 9 lines 27 et. seq.). Consequently, the process must be conducted at subatmospheric pressure. Suitable catalysts include compounds of Cu, Zn, Al, Sn, Ti, V, Fe, Co, and Ni. While it is disclosed as being desirable to dissolve the carbamate in a solvent, the process can be performed with the carbamate in the suspended or emulsified state (Col 8, lines 20 et. seq.). This patent does not disclose the thermolysis of polymeric aromatic carbamates or the thermolysis of aromatic carbamates at atmospheric or superatmospheric pressures.
U.S. Pat. No. 4,146,727 discloses a method for preparing dicarbamates and polymeric carbamates. In this patent it is suggested (See Col. 1 lines 25 et seq. and Col. 4 lines 56 et. seq.) that the polymeric carbamates described therein can be thermally decomposed in a solvent to their corresponding polymeric isocyanates in accordance with two of the aforenoted patents, namely, U.S. Pat. Nos. 3,919,279 and 3,962,302, notwithstanding the lack of detail in either of these two patents as described above, or the U.S. Pat. No. 4,146,727 patent, as to how this can be achieved.
U.S. Pat. No. 4,163,019 discloses a process for preparing 4,4'-alkylidene diphenyl diisocyanate by a two step process involving the condensation of a phenyl alkyl carbamate using an aldehyde or ketone to form a dimer, e.g., dicarbamate, and an exchange reaction wherein a phenyl isocyanate is mixed with the dicarbamate to form a phenyl alkyl carbamate and the corresponding diisocyanate. Certain tin compounds are disclosed as being suitable exchange catalysts. This reference does not disclose a use of these catalysts for the thermolytic cracking of carbamates in the absence of an exchange reaction.
An article in Chemical Week, Nov. 9, 1977, pp. 57-58 discloses a process which comprises the steps of reacting nitrobenzene, carbon monoxide and an alcohol to form corresponding urethanes (alkyl phenyl carbamates). The reaction product is reacted with formaldehyde to produce a condensate which contains p,p'-methylene diphenyl dialkylcarbamate and higher oligomers. This product is, in turn, thermally split into the corresponding "polymeric diisocyanates" and alcohol, which is recycled. The set of reactions is reported to involve the use of high temperatures in the range between 100.degree. and 200.degree. C. in the first reaction step and between 200.degree. and 300.degree. C. in the decomposition step and the reaction leads to a mixture of polymeric diisocyanates. This article does not disclose the use of the catalysts described herein for the present invention.
There are several difficulties which one encounters in attempting to conduct thermolysis of polymeric aromatic carbamates. Such polymeric materials are much less soluble in common solvents than non-polymerics. Consequently, even slight side reactions such as between isocyanate and carbamate reduce the solubility of the polymeric reaction product even further than would otherwise result from similar reactions using non-polymeric reactants. Once the polymeric material starts to insolubilize, the formation of tars, gums and other undesired by-products begins to accelerate. Low reaction temperatures also decrease the decomposition reaction rate requiring longer reaction times. Longer reaction times can provide more opportunity for undesirable side reactions to take place, although at a slower rate. If the reaction temperature is raised to increase the reaction rate and the solubility of the polymeric reactants and/or products and by-products, other undesirable side reactions begin to take place at an accelerated pace at these elevated temperatures. Furthermore, if the concentration of polymeric carbamate is too high in solution, the polymeric isocyanate product (which is non-volatile at reaction conditions and cannot be economically removed from the reaction medium by vaporization) will react more readily with the polymeric carbamate to form an allophonate which is even more insoluble than either the polymeric reactants or product isocyanate thereby destroying the reaction sequence. Diluting the reactants with solvent reduces the economic efficiency of the process and requires greater capital investment in plant equipment.
Consequently, a balance must be established between reaction temperature, and polymeric carbamate concentration and solubility to permit the process to be run economically. Accordingly and in view of the above there has been a continuing search for ways to reduce the decomposition reaction temperature of polymeric carbamates without sacrificing the reaction rate to any great extent or alternatively to increase the reaction rate at similar temperatures employed in the absence of a catalyst. A reduction in reaction temperature would decrease undesirable side reactions induced by more elevated temperatures. Increasing the reaction rate provides less time for undesirable side reactions to take place until product removal.
Regarding non-polymeric aromatic carbamates, the aforedescribed prior art clearly indicates that conventional disclosed reaction temperatures for the thermolysis of the carbamates to form the corresponding isocyanates varies from about 175.degree. to 350.degree. C. at atmospheric or supra atmospheric pressures. Accordingly, there has also been a continuing search for ways to either reduce reaction pyrolysis temperatures of non-polymeric aromatic carbamates below 175.degree. C. to reduce undesired condensation reactions which occur at elevated temperatures and thereby increase selectivity to the isocyanate, or alternatively to increase non-polymeric aromatic carbamate decomposition reaction rate at conventional pyrolysis temperatures to reduce the average reactant residence time in a reactor thereby permitting a reduction in capital investment in plant equipment (e.g. by reducing reactor size).
The present invention was developed in response to the aforedescribed searches.